Things that affect sound quality; your phone, your headphones/earphones and the most importantly the quality of the song you are playing. Mobile phones dont usually have good sound. The earphones that came with Lumias don't sound good but you can still do something about it, download higher quality songs and your phone will eventually sound better. If you have cash to spare you can get higher sound quality earphones for cheap, I suggest the cheapest sennheiser earphones as they sound really good.
Please note: Dolby Surround doesn't make the sound quality better, it only adds artificial effects. And I also noticed that older Nokia phones have better sound than the new Lumia phones